Quick! The boss is coming! Hide your screen! What, you don't have Spaces set up so you can switch to that handy Excel worksheet you've got open? That's okay -- just put your display to sleep.
Of course, there are other reasons to want to quickly turn off your display, but they all have one simple key combination: Control + Shift + Eject.
It works with both desktop and laptop Macs. Moving the mouse or pressing a key on the keyboard wakes it right back up when you're ready.
Of course, if you want to put your whole computer to sleep, you can use Option + Command + Eject. There are other helpful shortcuts, too, in this article deep in the TUAW archives.